To replace the natural white paper, a polyolefin synthetic paper based on polypropylene biaxially orientated film as the intermediate substrate layer, with its surface being laminated or coated with uniaxially oritated polypropylene films comprising inorganic salt fine powder as a paper-like surface has long been developed by Japan Yupo Corporation, and filed a patent (Japanese Patent No. 46-40794, Japanese Publication No. 56-141339 and Japanese Publication No. 56-118437), and commercialized. To improve the gravure printability of synthetic papers, Japanese Publication No. 50-10624, and Japanese Publication No. 50-161478 etc., discloses to coat 0.005˜0.1 g/m2 of acrylic copolymer or polyethylene imine solution on the synthetic paper surface; besides, Japanese Japanese Publication No. 3-87255 discloses that 35˜95% by weight of crystalline polypropylene with isotacticity above 98% and 65˜5% by weight of talc and/or mica powder are extruded and bi-axially orientated as a substrate layer (middle layer), and on at least one side of this substrate layer is laminated or coated another thin surface layer which is produced from an uniaxial orientation of a resin containing 20˜90% by weight of high crystalline polypropylene with 98% isotacticity and 80˜10% by weight of inorganic salt fine powder to constitute a composite resin synthetic paper with opacity more than 90%.
The synthetic papers of above Japanese Patent Publications show advantages of waterproof, tear resistance, rigidity, mechanical strength and adhesion to the printing head, however, the ink drying rate in the printing process is not so ideal that they are not widely applicable in writing and printing cultural paper. The reason is that the microvoid volume and the porosity connection degree of the synthetic paper are much less ideal than those of natural papers, resulting in that the ink drying time is quite long (4-color printing takes about 8 hours) after the printing process, it is inevitable to dry the ink on synthetic paper, also the amount of stacking is limited for preventing undesired reverse printing. As for the four-color double-sided printing, the number of printing operators is potentially needed to increase, besides, the working space to wait for drying is necessary, and because the polypropylene uniaxially orientated film structure containing inorganic salt fine powder exhibits worse ink adhesion amount than that of the natural paper, more ink should be put into the printing machine to achieve the color saturation; meanwhile, an unique printing ink at a 3-4 more times expensive price than that of the natural paper printing ink is needed to prevent the intrusion of the ink solvent resulting in paper deformation and dimensional change. In other words, the main reason that the synthetic paper of the polypropylene uniaxially orientated film structure containing inorganic salt fine powder can not widely replace the natural paper products in many applications is the long printing process and cost difference as compared with the natural paper.
In addition, Nisshinbo Industries, Inc. uses a surface treatment on the synthetic paper made of polypropylene, polyvinyl chloride, polystyrene or polyester film with or without orientation, although the characteristics (printability, drying property) of the resulted synthetic paper are much better than the above-mentioned synthetic paper made of polypropylene uniaxially orientated film containing inorganic salt fine powder as a paper-like surface layer, it should widely replace the cultural use of the natural paper, in fact, the manufacturing costs of this synthetic paper are quite high in various aspects, its sales in the market are not more ideal than the synthetic paper made of the polypropylene uniaxially orientated film containing inorganic salts fine powder as a paper-like surface layer, because its technology includes the use of DMF (Dimethyl formamide) solvent as a surface layer treating agent, the paper surface layer thickness after the coating is quite thick, thus the coating thickness is not less than 10 μm, the toxicity of the solvent is very strong, and the coating material hardens during processing, also an expensive wastewater treatment equipment is needed to be invested to treat the large volume of toxic waste water by-product in the foaming phase, otherwise it will cause environmental pollution problems.
For wide application of the synthetic paper to replace quality-wisely the use of the natural paper, meanwhile also avoiding the application scope narrower due to excessively high manufacturing costs, 15 years ago before the present invention application, the present applicant also provided a polypropylene resin composition through the co-extrusion of one primary extruder and two secondary extruders into the same T-die to make 3-layer polypropylene white opaque resin coated material sheet, and then after cooling, setting, biaxial orientation, corona treatment, the surface layer was coated with a paper-like surface processing agent containing inorganic salts in oily organic solvents (chlorinated polypropylene, benzene), depending on the thickness, to obtain a printing quick-drying type biaxial orientation polypropylene pearl shining synthetic paper of thickness 25˜250 μm (U.S. Pat. No. 6,364,988, application Ser. No. 09/394,914). Please refer to the following table for different zone—patent applications:
European Patent Application97110804.8 filed Jul. 1, 1997U.S. patent application Ser. No.09/131,833 filed Aug. 10, 1998Japanese Patent Application9-366839 filed Dec. 26, 1997
Because the microvoid volume and the porosity connection degree of the synthetic paper obtained by this invention are very close to those of natural paper products, thereby the drying time of the four-color printing ink is 10 minutes, while the drying time of the printed ink on the outer layer for avoiding the undersized reverse printing is less than one minute, so that the printing operation is entirely the same as that of natural papers, and the stacking volume control, printing plant workers increase and the waiting time for the drying operation are not necessary, besides, the ink absorbed is confined to the coating layer and does not invade into the substrate layer, therefore a general natural paper printing ink can be used; meanwhile its ink printing amount is similar to that of the natural paper, the ink volume put in the printing machine is not increased. It is thus clear that the qualities of the said inventive synthetic paper are beyond those of the generally known synthetic paper, and can widely replace the natural paper in the cultural paper application for printing and writing.
However, in the above applicant's invention, the inorganic salt compounds in the paper-like surface processing agent not only contain toxic chlorinated polypropylene, organic solvent benzene coating formulations, but also the synthetic paper process inevitably includes a 10˜25M drying oven equipment at temperature of 80° C.˜120° C., thereby, not only the plant and operating personnel, exposed to the solvent emission pollutants due to dryning, and industrial safety concerns this synthetic paper still contains residual traces of chlorine, benzene, for which the present applicants should re-rack their brains, conduct numerous experiments, review and improve, finally found that a biaxially orientated white opaque polypropylene three-layer structure substrate film (middle layer) is produced by one primary extruder using a resin composition composed of 90˜40% by weight of high crystallinity polypropylene with more than 97% isotacticity, 10˜40% by weight of calcium carbonate composite particles(45˜85 weight % calcium carbonate and 55˜15 weight % polypropylene resin), 0.1˜15% by weight of titanium dioxide composite particles(40˜60 weight % titanium dioxide and 60˜40 weight % polypropylene resin), 0.1˜5% by weight of antistatic agent, and 0.1˜2% by weight of ultraviolet absorber; on the upper surface and the lower surface of the middle layer are co-extruded with the resin composition composed of 50˜99% polyethylene and 0.1˜5% anti-blocking agent by two secondary extruders, three extruders coextrude the molten material into the same T-type die to form a three layer coated material sheet, after the biaxial orientation and the corona treatment, a paper surface processing agent composed of 8˜20% by weight of acrylic resin, 20˜60% by weight of calcium carbonate, 0.1˜5% by weight of clay, 0.1˜2% by weight of titanium dioxide, 30˜90% by weight of water and 0˜2% by weight of antistatic agent is coated with a gravure coating roller to form a coating thickness less than 10 μm, resulted in a printing quick-drying environmentally friendly biaxially orientated polypropylene synthetic paper, and thereby the target of the present invention is realized.